1. Field of the Invention
This invention relates to the field of sintering powder parts and particularly to the sintering of pre-shaped compacts made from Si.sub.3 N.sub.4 ceramic alloy compacts.
2. Description of the Prior Art
Pressed and sintered parts made from Si.sub.3 N.sub.4 powders with or without additives such as metal oxides are useful for some high temperature structural applications. The pressed powder compacts must be protected from passive oxidation during the high temperature (approximately 1700.degree. C.) sintering process. At the sintering temperature, Si.sub.3 N.sub.4 oxidizes to SiO.sub.2 if the partial pressure of oxygen in the furnace atmosphere is greater than about 0.1 atmosphere, according to the following: EQU Si.sub.3 N.sub.4 (s)+3O.sub.2 (g).fwdarw.3SiO.sub.2 (s,1)+2N.sub.2 (g)
Such passive oxidation is undesirable and increases the weight of the sintered compact.
To protect the compacts from such passive oxidation, non-oxidizing furnace atmospheres are used.
Typically, a nitrogen atmosphere is used with an oxygen partial pressure as low as possible. In some instances, compacts are packed in their own powder and sintered in a nitrogen atmosphere, however a nitrogen atmosphere is still considered essential. The use of a nitrogen atmosphere or other controlled atmosphere furnace adds unwanted complications, and makes the sintering of Si.sub.3 N.sub.4 components a batch operation.
A particularly useful Si.sub.3 N.sub.4 ceramic alloy is Si.sub.3 N.sub.4 -CeO.sub.2. Prior art attempts to densify this alloy by sintering in nitrogen at one atmosphere pressure were unsuccessful due to a volatilization phenomenon. It has been reported (H. F. Priest, G. L. Priest, and G. E. Gazza, J. Am. Ceram. Soc. 60, 1977) that Si.sub.3 N.sub.4 +20 weight % CeO.sub.2 composite powders can be densified by sintering at 1800.degree.-1900.degree. C. in nitrogen at pressures between 250-300 psi. Such high nitrogen pressures are difficult to obtain at the sintering temperature and virtually exclude such prior art method from commercial application.
Hot pressing has been used to densify Si.sub.3 N.sub.4 +20 weight % CeO.sub.2 and the resulting samples had an average flexural strength of 100,000 psi at room temperature and 80,000 psi at 1400.degree. C. Unfortunately, complex-shaped articles cannot be manufactured by hot pressing.